To date, particularly when an available “soft” elastomer is to be directly overmolded onto a shaped article of nylon, as typically done when insert-molding an OM layer onto the article, the composition of the elastomer must be tailored to the particular nylon substrate, depending upon the molecular structure of the nylon. For example, when the nylon is Ultramid® 1703-2, a 25% glass reinforced 6,6-nylon, it is necessary to formulate a specific combination of components for an OM elastomer layer which will adhere acceptably and serve the desired function on this particular nylon; when the substrate is Ultramid® A3WG6 BK90564, also a 6,6-nylon, but a different molecular weight and with 30% glass fiber, it is necessary to formulate a different combination of components for an OM elastomer layer which will adhere acceptably and serve the same purpose. The differences in formulation are magnified when the differences in the nylons is greater.
As used herein, the term “nylon” is a generic term for any long-chain synthetic polymeric amide which has recurring amide groups as an integral part of the main polymer chain. To date, before the invention disclosed herein, since it is not easy to determine the Mw and particular repeating unit from which commercial nylon a shaped article was thermoformed or otherwise crafted, the process by which one arrived at a suitable self-adherent, “good-feel” OM TPE composition was by extensive trial and error.
Moreover, even when one skilled in the art conventionally formulated a OM TPE for nylon, using a “SHDS” rubber such as Kraton FG19901 with a functionality in the range from about 1.5-2%, (“SHDS” is used for brevity, to denote an at least partially hydrogenated “styrene-hydrogenated diene-styrene” which is preferably fully hydrogenated) the OM layer failed to bond satisfactorily even at high molding temperatures, particularly for insert molding, for the nylon substrates used herein. The acronym “SDS” (for styrene-diene-styrene) refers to a linear polystyrene-conjugated diene (“polydiene”)-polystyrene block copolymer, polydiene typically referring to polybutadiene and/or polyisoprene in the midblock, but the acronym broadly refers to a SDS derived from a monovinyl aromatic and a conjugated diene which monomers may be mixed with other structurally related co-monomers, e.g., styrene as main aromatic component and a minor amount of α-methylstyrene.
The polydiene midblock, typically of butadiene or isoprene, gives the polymer its rubbery properties, while the polystyrene or poly(α-methylstyrene) blocks constitute the thermoplastic phase. Because the polydiene block contains double bonds (aliphatic unsaturations) which are oxidation sensitive, the TPE preferably uses hydrogenated butadiene or isoprene units, or both, so that if the TPE is an SBS (styrene-butadiene-styrene) block copolymer prior to hydrogenation, the resulting hydrogenated (styrene-ethylene/butylene-styrene) block copolymer typically has less than 5% unsaturation, preferably less than 2%. Analogously, if the TPE is an SIBS (styrene-isoprene/butadiene-styrene) block copolymer prior to hydrogenation, the resulting hydrogenated TPE is a SEEPS (styrene-ethylene/ethylene/propylene-styrene) block copolymer. The foregoing SHDS include “high vinyl” SHDS which are substantially fully hydrogenated. By “high-vinyl” is meant that at least 51 mole % (percent) of the butadiene midblock is polymerized at the 1,2-position, and at least 51 mole % of the isoprene, if present, is polymerized at the 3,4-position by “driving” the polymerization with addition of a polar compound, as is well known in the art; typically the maximum in each case is 90 mole %. Such HSBCs are referred to as “high vinyl” HSBCs whether either butadiene or isoprene, or both, are present in the midblock.
There is a particular need for an OM TPE which will adhere and remain tightly adhered to a surface of a solid, predominantly nylon shaped article, irrespective of the type of polyamide from which the nylon is derived, or the additives packaged with the nylon, or the process conditions of its molding, or its aging history, and irrespective of whether the OM layer is molded by “injection overmolding” also referred to as “insert molding”; or, by “two-shot injection molding”; or, coextrusion with a nylon substrate; or, by multilayer blowmolding over a nylon substrate.
Among the more commercially relevant thermoplastic elastomers are those based on physical blends of polyolefins and rubbers, and particularly, blends of TPEs in which blends a polyamide was used to develop the bond required to securely fix the OM layer to the substrate.
For example, in U.S. Pat. No. 5,750,268 to Mace et al, a blend they made for an OM layer, required from 5 to 50% by weight (“by wt”) of a “polar engineering thermoplast”, referring to PA6, a 6,6-nylon.
Consistent with the belief that the polyamide was an essential component of a OM layer, U.S. Pat. No. 5,843,577 to Ouhadi et al. discloses a blend of a Santoprene® rubber, and the reaction product of a functionalized polyolefin with a polyamide, relying on the presence of the polyamide structure in the composition, to generate a strong bond when the molten blend comes into molding contact with a nylon substrate.
In contrast to the foregoing, the OM layer disclosed herein particularly relates to a TPE comprising a polyamide-free blend of block copolymers uniquely adapted to be overmolded on, and tightly adhered to any solid predominantly polyamide (“nylon”) surface irrespective of the particular amide repeating unit of the nylon. The blend is particularly adapted for use in insert molding where a substrate to be over-molded is introduced at about ambient temperature at which adhesion to the surface is far more difficult than in two-shot molding where the substrate is barely solidified. The novel polyamide-free blend may be tailored to provide an OM layer at either low, average or hot temperature profiles in the barrel of an extruder or injection molding machine. It is found that the novel TPE elastomer is effective to bond to a large variety of polar substrates without the use of an adhesive between the substrate and the OM composition forming a layer.
In the recent past, wood or metal handles on a variety of hand tools such as screwdrivers, handles for power tools such as circular handsaws, and portions of containers in which tools are housed, have all been replaced with injection-molded nylon handles. The type of nylon varies from one tool to another for a variety of reasons, one of which is to accommodate the particular rigid core element to be sheathed in the nylon. Some nylons are molded over a steel shaft, as in a screwdriver or butcher's knife; some nylons are molded over a metal door handle or latch handle; some handles are made from glass fiber reinforced nylons, by injection molding. To date, irrespective of the particular nylon gripping means, all have the drawback of being “hard” on a person's hand, that is, they provide no cushioning whatsoever against impact or vibration. Such nylon gripping means are no more “user-friendly” than wood, and far less user-friendly than metal.
One approach to “soften” a handle is to cover the handle with a soft, thin, flexible cover of a foamed synthetic resinous material. Another is to overmold a soft elastomer onto the nylon, using an insert-molding procedure, such as is now commercially done with the OM6000+ series of overmolding compositions provided by GLS Corporation. Prior art OM layers used a “higher functional” SHDS rubber without regard to the effect of morphological characteristics of the rubber, specifically its ability to “wet” the nylon substrate at temperatures of about 540° F. or lower, typically 360° F.-500° F.
The received wisdom is that the higher the functionality of the SHDS, the higher the statistical chance of having functional groups react with the nylon substrate. However, such “higher functional” SHDS rubbers, that is, having a functionality appreciably greater than 1%, e.g. typically 2% or more, do not adequately “wet” the substrate even if the substrate is heated by contact with the OM layer, because amide groups in the substrate are not heat-activated sufficiently to react with the functional groups on the rubber.
Quite unexpectedly, judging from the higher bonding (peel) strengths, better wetting is obtained with a hot OM composition containing a “lower functional” SHDS having a functionality of from 1% but less than 2%, when the amide groups are similarly heated.
For convenience and brevity, and also to avoid the non-specific nature of the term “lower”, a SHDS having essentially 1% functionality is referred to hereinafter as “unifunctional”; and a SHDS having from >1% but <2% functionality is referred to hereinafter as “higher functional” since functionality of 2% and higher is ineffective for the purpose at hand.
Good bonding with unifunctional and higher functional SHDS contradicted the prior belief, namely, that the higher the functionality of the rubber, the better the chances of reactions between the amide groups on the substrate and the functional groups in the rubber. That belief ignored the effect that the structure and morphology of the heated functionalized rubber will have on the nylon; it also ignored the requirement of “small end blocks” or “high rubber” SHDS (the terms are used synonymously) to provide the “right” structure of polymer chains organized in a morphology that allows the functional groups on the rubber to be contiguous to the amide groups, in the first place, so they can react at the surface more easily.
The foregoing considerations and numerous relevant prior art references which routinely teach addition of mineral fillers in SHDS compositions, also failed to recognize that presence of an inert filler might be critical to establish the desired bond. Understandably, no weight was accorded in the prior art, to the presence of a filler having no noticeable adhesive qualities, per se, in an OM layer. There was no reason to expect that a particulate, optionally fibrous, filler present in a defined concentration range in a SHDS composition, would be critical to provide a “filled” OM layer with necessary excellent adhesion to any nylon even at insert-molding conditions, without which adhesion no OM layer is useful from a practical point of view.